1. Field of the Invention
The invention pertains to a construction technique for a new and improved electromagnetic measuring system. In particular, the invention describes a technique for greatly increasing the sensitivity while reducing the size of Josephson Junction Cryogenic Magnetometers.
2. Description of Prior Art
Cryogenic magnetometry is a new but rapidly developing field of technology. Although the sensing device itself can be made quite small, it is designed to function at temperatures a few degrees above absolute zero and so must be operated within large bulky Dewars containing liquid helium coolant. The operation of the devices depends on the effect of an applied magnetic field upon the superconducting properties of Josephson Junctions. A Josephson Junction is commonly described as a `weak link` or tiny barrier separating two bulk superconductors and may be a dielectric barrier on the order of 50A.degree. thick, a tiny constriction or a point contact. The amplitude of the supercurrent across the junction is a function of the quantum mechanical phase difference between the Cooper pair electron wave functions across the junction, and the phase difference is, in turn, dependent on the applied magnetic field. Application of these effects in magnetometry has followed two main lines of development. The most highly developed system is the single junction RF superconducting quantum interference device or SQUID. This device is equivalent to a superconducting ring having a single weak link coupled to a resonant circuit driven by a constant current source at a selected RF frequency. Both the Q-factor and the resonant frequency of the circuit are modified by the coupling to the SQUID dependent upon the magnetic flux through the ring. The other line of development is the two-junction DC SQUID in which a superconducting loop incorporates two junctions in parallel. For the DC SQUID, the maximum supercurrent across the device, the critical current, is a periodic function of the magnetic flux enclosed in the loop. A DC SQUID is usually operated in a resistive mode at constant current in which the total current is due in part to superconducting electrons and in part to normal electrons. A voltage signal is then picked off a convenient operating point of the corresponding current-voltage curve. Changes in this voltage are a function of changes in the magnetic flux contained within the loop. The described invention is a DC SQUID.
A further problem is that instruments for measuring magnetic fields are generally bulky and complex, especially where the magnetic field or magnetic field gradient is very minute. A SQUID is essentially a vector instrument and thus sensitive to rotation. However, complex, multi-SQUID arrays, each SQUID in the array separated by some distance from other SQUIDS in the array and mounted on a rigid substrate, can theoretically be made insensitive to rotation and used to measure magnetic field gradients, curvatures and other higher-order derivatives of the field. Large arrays, however, become impractical very quickly due to the size and weight of the Dewars necessary to contain the liquid helium. If the sensitivity of a conventional SQUID were to be increased by a facor of two, the volume and weight of a gradiometer comprised of three mutually orthogonal SQUID arrays could be decreased by a factor of eight for sensitivity equivalent to a larger conventional array. Significant advances in inreasing the sensitivity of SQUIDS would yield advances in instrumentation arts of great importance.
Yet another limitation possessed by conventional magnetometer construction is that conventional SQUIDS themselves generate magnetic fields or, in some instances, disturb the applied magnetic field such that a plurality of SQUIDS arrayed in close proximity to one another experience mutual interference as the result of positive coupling (mutual inductance). Thus most prior art devices use only single loop SQUIDS. The described invention allows closely packed multiple loop SQUID arrays yielding much greater sensitivity in far less volume with negative noise coupling and subsequently decreased mutual interference or noise signal voltages.